This invention relates to radiation curable adhesives and a process for gluing optical glass components using these adhesives.
For gluing optical glass components, for example, optical lenses or, in particular, also glass plates in the production of liquid crystal display cells, it is necessary to have available adhesives which are suitable for this purpose. It should be possible to harden such adhesives in the minimum amount of time in order to avoid long fixing times of the components during the gluing operation. The hardened glue should be transparent, have a high mechanical strength and be resistant towards weathering influences. In the case of the liquid crystal display cells, it should also be resistant towards the contents of the liquid crystal cells.
These properties are currently best fulfilled by adhesives which can be hardened by radiation and are based on ionically polymerizable epoxide systems and ionic photoinitiators of the triarylsulfonium complex salt type. Corresponding compositions which can be hardened by radiation are known, for example, from U.S. Pat. Nos. 1,516,511; 1,516,512; 4,058,401; 4,138,255; 4,161,478; 4,173,476; 4,186,108 and 4,218,531 and many other publications. The use of such compositions for gluing glass plates, optical lenses or liquid crystal display cells is moreover described in German Offenlegungsschriften DE-OS No. 2,518,652 and DE-OS No. 2,904,626 and in U.S. Pat. No. 4,297,401.
For optical glass components, such as lenses or liquid crystal display cells, especially if these are to be produced with high precision, it is necessary that they can be adjusted during gluing, that is to say can be brought into their exact position relative to one another. Adjustment before, after or, most advantageously, during the gluing operation should be possible. With state of the art adhesives which can be hardened by radiation--their typical exposure times required for hardenng are in the range from seconds to a few minutes--it is not possible to correct the position during or after gluing. The hardening time may be too short, or the adhesive layer is thereby irreversibly destroyed when it has not yet completely hardened, or the components are moved back again to the starting position by the partially hardened adhesive after their position has been corrected. When such adhesives are used, adjustment of the component to be glued is only possible before the hardening by radiation, expensive adjustment equipment being required while the components are brought together and held in the exact position.
Ester/ether optical adhesives which can be hardened by light and are based on certain dioxolanylmethyl(meth)acrylates, and which contain both photoinitiators which form free radicals and cationic photoinitiators and which can therefore be polymerized step-wise, are described in German Offenlegungsschrift No. 3,323,733. It is said to be possible, using such adhesives, initially to fix optical components by UV radiation and then to harden the gluing by heat treatment or further UV irradiation. These materials also do not yet fulfill the current requirements of an efficient optical adhesive. On the one hand, relatively long irradiation times are required for the partial hardening--typically 30 to 120 seconds and very long heating times of about 5 hours or subsequent irradiation times of 2 to 10 minutes are required for complete hardening. On the other hand, the position of the fixed glass components can be corrected only by heat treatment or by dissolving or partial swelling with the aid of solvents. The time span for adjustment in the pre-hardened state is also limited to about 5 to 6 minutes.